It is conventional practice in gear couplings having gear synchronization to achieve synchronization of the shaft and gear to be coupled by the use of synchronizing rings which are axially shifted through a gearshift sleeve. The synchronizing rings are provided with internal or external conical friction surfaces which engage corresponding conical surfaces of the shaft or gear with the resulting frictional engagement establishing the desired synchronization of the parts involved. Such an assembly is shown in the German Pat. No. 1,500,332.
Also, gear clutches are known wherein the synchronizing rings have both internal and external friction surfaces, as shown in German Pat. No. 1,550,172. These synchronizing rings are provided for the locking synchronization of transmission change gears, especially in automotive vehicles.
In general, synchronizing rings are formed of materials having properties which are designed for the demands placed on the synchronizing rings. One frequently used material is a brass alloy which, having the requisite surface tooling on the friction surface, provides the coefficient of friction required for effective frictional engagement as well as the strength required for the gearing. However, these alloys have the disadvantage of being expensive and are limited in their performance capacity. Also, the alloys are difficult to work with in the manufacture of the rings and entail considerable expense.
Synchronizer rings of ferrous metal, especially sintered metal, are simple to manufacture and have the advantage of lower tooling costs. However, the sintered material does not have a good coefficient of friction due to the absorption of oil present in the gear by the material. For this reason, special alloys of sintered material have been tried to obtain better coefficients of friction, however, these alloys like the special brass alloys are also expensive.
To provide improved synchronizing rings, ring bodies have been produced from sintered metal with the friction surface for the ring provided by spraying a layer of molybdenum on the ring, such as described in German Patent Specification OS No. 2,055,345. Radially running transverse grooves were machined into the molybdenum friction surface to allow for the flow of oil from the ring. Although the synchronizing rings produced by this process had friction surfaces with high coefficients of friction, the rings did not exhibit the desired frictional characteristics or have the desired life. Furthermore, the manufacturing costs associated with this process are extremely high such that the synchronizing rings were very expensive.
In order to avoid imperfections in the molybdenum layer, it was proposed in German Specification OS No. 2,055,345 to make the annular body with trough shaped depressions and then to apply the molybdenum layer in such a way that local weaknesses are not present in the layer. The molybdenum layer is applied to have a constant thickness in the depressions as well as on the remainder of the friction surface. However, the manufacturing technology of this process is extremely expensive.
Synchronizing rings with a molybdenum friction layer have the disadvantages of insufficient strength in the frictional engagement during synchronization to provide the shifting operation with a low shifting force at high gear performances and of a relatively short life of the synchronizing ring or its molybdenum layer. To avoid these disadvantages, it was proposed to enrich the molybdenum layer with oxygen to increase the frictional effect. This addition, of course, further increases the manufacturing costs.
Therefore, it is evident from the above proposals that to fulfill the requirements for present gear shift arrangements, the synchronizing rings can only be produced by means of relatively expensive manufacturing processes. Other forms of synchronizing rings have the disadvantage of a short useful life or that the frictional characteristics and coefficient of friction will decrease with increasing age. Decrease in the frictional properties leads to poor synchronizing effectiveness which is associated with considerable shifting noises.